• Single Responsibility Principle:

  Don’t put all your worker eggs in a single basket. A class should have one reason to change.

• Open/Closed Principle:

  Don’t change the same class over and over. If you find this happening, abstract out what is changing.

• Listov Substitution Principle:

  Return the same type in an overridden subclass method as a parent class method. The same applies for a method’s parameters. Be consistent.

• Interface Segregation Principle:

  Don’t create interfaces with many (more than five) methods. This is a sign that you’re doing too much in one place.

• Dependency Inversion Principle:

  Rely on interfaces and abstract classes more than concrete classes. This will be more flexible.

Encapsulate What Varies:

Only abstract away things that vary in your application. For example, if a Mailer class will never change, don’t get hung up on writing a lot of abstraction around that: focus on what will change.